In lamps such as gas-discharge lamps or halogen lamps, the body of the lamp is often made of quartz glass and encloses a burner or chamber with a filling. In the case of lamps such as a high-intensity discharge (HID) lamp, the fill gas or filling can comprise an inert gas as well as various metal salts. The electrodes, usually embedded in a sealed portion of the lamp, can become very hot on account of the high current that flows through the electrode during switch-on and during operation of the lamp. The hot electrodes cause the quartz glass to also heat up. The different coefficients of thermal expansion of the quartz glass and the electrode metal lead mean that these expand and contract at different rates during heating and cooling respectively. A known problem caused by these different expansion and contraction rates is that cracks appear in the quartz glass, since quartz glass expands and contracts to a lesser extent than metal. During the lifetime of the lamp, the cracks can become larger. For example, a number of small cracks can spread and join to form an enclosed region in the sealed portion in the form of a ‘bead’. Also, one or more small cracks can develop into a crack extending radially outward, known as a ‘radially extending crack’ or REC. A bead-like crack can also develop into a REC. Such cracks can ultimately lead to failure of the lamp or, in a worst case, to explosion of the lamp.
Much effort has been invested in finding a solution to the problem of lamp failure due to cracks in the sealed portion, since long lifetime and reliability of performance are extremely important factors, particularly in the case of gas-discharge lamps used for automotive purposes. Some efforts describe wrapping a coil around the electrode in the region that will be enclosed in the sealed portion. Such techniques are time-consuming and expensive, and therefore not practicable. In one approach, US 2007/0103081 describes an electrode treated so that cracks largely develop in a controlled manner. This document teaches the treatment of the electrode, for example by creating a deep pit in the electrode or a deep groove around the body of the electrode, in order to deliberately allow a bead-like crack to develop during operation of the lamp. However, observations have shown that such grooves in the electrodes are associated with a high proportion of electrode breakages, even during the manufacturing process, so that this type of electrode treatment is not particularly advantageous from the point of view of a prolonged lamp lifetime as well as a desirable production yield.
Other approaches are based on minimizing the contact between the electrode body and the quartz glass in the sealed portion. For example, WO 2008/032247 describes electrodes treated so that bristle-like protrusions, arranged in a spiral manner on the sides of grooves running around the electrode, result in a separation between quartz glass and electrode in a critical part of the sealed portion that is most subject to extreme temperatures during operation of the lamp. However, this type of electrode is also associated with an undesirably high failure rate, resulting in shorter life-time and an undesirably low production yield. The reason for this is that, to reach the necessary high pressure of inert gas in the lamp, a cooling step is required during manufacture. Cooling is carried out rapidly, for example by immersing the seal (or the entire lamp) in a liquid nitrogen bath. This means that, in the sealed portion, the quartz glass and the electrode both contract, but the electrode contracts to a greater extent. While the electrode contracts, axial forces are exerted on the electrode, which arise as a result of the adherence between the quartz glass and the electrode in the sealed portions on either side of the critical region. In effect, the grooved region is being held firmly at both ends, while at the same time being forced to contract. The relatively deep groove in the body of the electrode causes the electrode to behave as a notched tensile specimen. Often, this results in the groove developing into a crack or break in the body of the electrode during cooling, and the lamp is rendered useless. The same applies to US 2007/0103081, since any deep pit or groove in the body of the electrode increases the likelihood of failure during cooling.
It is therefore an object of the invention to provide an improved electrode which reduces the lifetime-related and production-related problems outlined above.